Detecting hyperthreading has to be done from assembler, and I'm not aware of any Win32 API that provides that information. There is an article on the Intel website that explains the intracacies.

Update: Here's an implementation in D. It should be trivial to convert to C.

import std.stream;
import std.stdio;

void main ( ) {
    int ht_supported = 0;
    int cores_per_socket = 1;
    asm{
        mov EAX, 0;
        cpuid;
        test EAX, 0x10000000;
        jz NO_HYPER;
        mov ht_supported, 1;
        mov EAX, 4;
        cpuid;
        and EAX, 0xfc000000;    
        shr EAX, 26;
        add EAX, 1;
        mov cores_per_socket, EAX;

    NO_HYPER:;
        
    };
    
    if( ht_supported ) {
        printf( "Hyperthreading is supported and there are %d cores pe
+r cpu\n", 
            cores_per_socket );
    }
    else {
        printf( "Hyperthreading is not supported on this processor\n" 
+);
    }
}
[download]

Update2: I also turned up this quite comprehensive cpu features detector sample on MSDN